Nothing/Anything. Before you buy an
add-on serial port card, make sure you really need it. Embarrassingly
often, we've popped the lid on a system intending to
install a serial port only to find that there was already a perfectly
good one unused. In our defense, those little DB9 connectors are easy
to miss in the tangle of cables on the back panel, and if the port is
disabled in the BIOS, Windows doesn't report it as
present.
So, first make sure you don't already have what you
need. If it turns out that you do need to install one, think again.
Serial ports consume a lot of resources for the functionality they
provide. You may be better off installing an inexpensive A-B serial
switch and sharing one port between two devices, for example an
external modem and a Palm cradle.
If you absolutely need to add a serial port, buy one of the
inexpensive (~$20) serial port cards commonly sold by computer stores
and mass marketers. We've used many different ones,
all of which seem to work fine and none of which is noticeably better
than any of the others.